Because of its enormous scope in virtually all industrial processes, any significant improvement in heat transfer from one fluid to another and evaporation of aqueous liquids cannot help but be of utmost importance to most industries. Such improvement is concerned with innumerable processes, some of which are: making good water from seawater and brackish water, salvaging the mineral content, if possible; converting industrial waste waters to potable water, salvaging the contaminants, if possible; converting radioactive atomic waste waters to solids which may be more readily isolated or stored in harmless areas than is possible with liquids; evaporating juices such as in the cane and beet sugar industry more efficiently with consumption of less energy for such; converting acidic mine waters to harmless solids, salvaging the water and valuable mineral content; converting brines, such as Great Salt Lake, into fresh water and valuable mineral contituents; and workable heat transfer systems to utilize steam power and recover valuable mineral contents from steam and brine produced by natural geothermal wells. Further developments in connection with fluids derived from geothermic wells are set forth in my application Ser. No. 588,797 filed June 20, 1975 entitled HEAT EXCHANGE METHOD AND APPARATUS, which application is a continuation-in-part of Ser. No. 306,183, now U.S. Pat. No. 3,891,496 dated June 24, 1975 and of the present application Ser. No. 581,849. In all of the aforementioned processes, the fouling of heat transfer surfaces and surfaces of confining vessels and the prevention thereof is astronomical in cost in manpower, maintenance, downtime, and low energy efficiency. This invention offers a novel, practicable means and method for prevention of such surface contamination without need of expensive and elaborate chemical treatment, together with novel means and methods for handling and separation of difficult mixtures of solids, liquids and gases.
In my U.S. Pat. No. 2,903,243, granted Sept. 8, 1959, is disclosed my discovery that precoating a heating element with a preferentially oil wettable material such as a silicone, epoxy resin, or phenolic resin prevented the adherence of insoluble calcium carbonate and other precipitates to the element when heating a mixture of crude petroleum and mineralized water, wherein the element was continuously exposed to an oil wash. Even though these coating materials proved fully effective in scale prevention at relatively high temperatures where no boiling occurred in heat treating crude petroleum emulsions, I later learned they would not apply to systems boiling mineralized waters. I also found that the reason the above-mentioned coating materials would not prevent scaling at boiling temperatures was because they adsorb water after prolonged contact with it, and become water wettable and susceptible to solids adherence. This is very prevalent when saturation is reached, and soluble salts are precipitated. Most industrial plastics such as epoxy resins, phenolic resins, polyethylene, neoprene, and the silicones are generally regarded as being preferentially oil wettable, but since they all adsorb traces of water upon prolonged contact with it, they eventually become oil repellant when immersed in a water environment, becoming water wettable and subject to solids adherence. Although I am not familiar with all industrial plastics, I have found only the fluorocarbon resins such as DuPont's Teflon to be substantially zero water adsorbent and permanently water repellent when immersed in water indefinitely. I would presume pure gold to also be zero water adsorbent, since it has been found that a thin gold coating will render steam condensing tubes permanently water repellant and subject to dropwise condensation. In view of the foregoing, I have discovered that zero water adsorbent coatings such as the fluorocarbons, particularly DuPont's `hexafluoropropylenetetrafluoreothylene` with the tradename FEP, retain their preferentially oil wettable properties in indefinite contact with boiling brines, and that their wetting with oil effectively prevents adherence of any precipitated salts to their surface. I also tried similar tests, using an oil wash, with phenolic resin and epoxy resin coated heating tubes and found the salt growth on them to be quite rapid, as much as 1/2 inch salt thickness on a 1 inch diameter tube in 8 hours boiling of brine. FEP coated heating elements also salted up in boiling brine when no oil was present in the system. The teaching in my patent U.S. Pat. No. 2,903,243 and other patent art to the present time, and in technical literature in general, is not sufficient to enable technicians to boil brines or mineralized waters without scaling, even though the so-called preferentially oil-wettable materials listed in U.S. Pat. No. 2,903,243 are used with oil. If it were so, this means would have been employed years ago to cope with this very serious problem. Only in comparatively recent work, embodied in this and companion applications, have I discovered that the vital key is the use of substantially zero water adsorbent coatings, rather than the general run of temporarily oil wettable coatings. A chief discouraging factor in the use of fluorocarbon coatings on metallic heat exchanger tubes has been the prohibitive cost and difficulties in achieving good, permanent coatings; so difficult that industrial coaters shy away from doing such. Pure, small diameter, comparatively thick walled (10 mils) bundles of teflon tubes are in use for corrosive fluids, but they do not teach the use of oil films with them to prevent fouling. A metallic thin walled tube with a very thin coating of fluorocarbon (range of 0.5 to 2 mil thickness) is best for efficient heat transfer. Because of urgent need for such a tube to effect the method disclosed in this application and companion applications, I have perfected an economical means of heat bonding a coating of DuPont FEP to a metal surface which is strongly bonded and substantially permanently zero water adsorbent, and suitably thin. This method is the substance of my pending application Ser. No. 41,375, now abandoned, and in my U.S. Pat. Nos. 3,837,956 dated Sept. 24, 1974 and 3,841,937 dated Oct. 15, 1974, both based on said application Ser. No. 41,375. I believe the novel disclosures on new and important uses for the basic methods described in U.S. Pat. No. 2,903,243 in this application, together with my discovery that substantially zero water adsorbent materials are essential to boiling of mineralized waters, are distinct and major improvements over the teaching of U.S. Pat. No. 2,903,243. I further believe that they will prove to be vital elements in solving some of the presently unsolved very serious problems in a broad spectrum of industry.
The Objects and Advantages of this Invention are as follows: Provision of method and apparatus to:
1. Exchange heat with maximum continuous efficiency between two fluid systems through a permanently oil wettable wall without deposition of solids on the wall, wherein one of the fluids is, substantially, a mixture of oil and aqueous solution, and the other fluid may be liquid or gas or both, containing oil if solids are present,
2. Efficiently and economically waters containing dissolved and/or suspended solids to any desired extent including total dryness; recovering both the solids and water vapor or only either, if desirable; all, without fouling of the heating and evaporating system with mineral scale or solids.
3. Accomplish (1) and (2) without chemical treatment to prevent fouling of heat transfer surfaces thereby eliminating chemical costs and expensive chemical control.
4. To convey or circulate liquids containing dissolved or suspended solids without deposition of solids on the contacting surfaces thereby avoiding expensive shut-down time for clean-outs.
5. To heat, cool or evaporate extremely corrosive liquids, acidic or alkaline, in (1) and (2).
6. To accomplish (2) in multi-stage or vapor compression distillation if required thereby conserving energy.
7. Provide a novel, permanently oil wettable heat exchanger tube design for heating mixtures of oil and solids-forming aqueous solutions wherein a vertically disposed tube is attached to the exchanger header at one end only, permitting uninhibited thermal expansion when heated; avoiding sedimentation common to horizontally disposed heat exchangers and vertical heat exchangers with conventional headers at each end of the tubes; permitting simple removal of the tube bundle from the top of the heating vessel without drainage of the vessel. Further provide an inner return condensate tube inside the heat exchanger tube with a unique arrangement of sizing that permits complete and continuous removal of liquid condensate from the lower portion of the heat exchanger tube with a negligible loss of vapor assuring maximum heat transfer efficiency with constant condensate removal.
8. Enhance heat transfer by the method of injecting inert gas or steam into the heat transfer zone outside the heating tube, wherein the injected gas increases the flow velocity of the circulating liquid and disrupts stagnant, insulating films on the heater tube, increasing heat transfer rate as much as 80% thereby greatly reducing the size requirements for expensive equipment.
9. Prevent salt buildup in and seizing of pump with water wettable interiors when circulating oil is contaminated with salts and brine, by injection of fresh water into the oil stream to the pump suction in amounts as small as one part of water to 3000 parts of oil; thus avoiding the need for a very expensive step of complete cleansing of the oil prior to pumping, or the alternative expense of providing of permanently oil wettable interiors for the pump.
10. Feed an aqueous solution (brine etc.) to a circulating hot oil as a highly dispersed mixture composed of the solution and oil from a cooler part of the system, this mixture being effected by a mixing pump; with oil being the continuous phase of the mixture in order to facilitate complete mixing with the circulating hot oil when combined with it, which, in turn, provides thorough dispersion of the solution droplets throughout the hot circulating oil without the added cost of having to subject the hot circulating oil to mixing with the aqueous solution in a pump with attendant vapor lack problems; the purpose of the whole procedure being to more efficiently vaporize the solution droplets by intimate contact with the hot oil with minimum pump size.
11. Assure oil wettable surfaces for all interior areas contacting the fluids being evaporated or circulated, using substantially zero water adsorbent materials (such as fluorocarbon resins) on heating surfaces, heater chamber interior, cascading baffles, and interior of flashing chamber; and lesser oil-wettable materials such as neoprene for interiors of a hot oil-aqueous solution mixing chamber, and conduits between the evaporator tank and the heating chamber; also using substantially zero water adsorbent surfaces contacting the solids slurry throughout its flowsheet all aimed at trouble-free operation.
12. Prevent solids crystalline growth on roughened surfaces or sharp edges, even though permanently oil-wettable, by complete elimination of such surfaces or edges from the hot oil-aqueous solution mixing zone to the solids slurry settling zone in the flashing cascading chamber providing for more trouble-free operation than otherwise attainable.
13. Assure a more efficient and less fouling operation with abundent surface exposure of liquid-vaporsolids mixture in the cascading chamber by cascading it downward over tiers of inverted, half cylinders which are preferentially oil wettable, and which contain no sharp edges or rough surface; which, further, permit removal of released vapors through their ends without recontacting them with the falling liquid.
14. Effect a smoother and more efficient operation by automatically and separately removing produced solids slurry and unevaporated aqueous solution as they accumulate.
15. Recycle the unevaporated aqueous solution, if required, admitting sufficient oil with the solution to prevent solids deposition on the preferentially oil wettable interiors of the pump and conduit for its recycling providing fouling-free operation.
16. Convey the unevaporated aqueous solution remainder, if required, to a 2nd stage evaporator, injecting sufficient hot oil from this 2nd stage into the solution as it leaves the 1st stage to prevent solids deposition on all conduit and controls interiors which are preferentially oil wettable.
17. Remove mist of entrained oil suspended solids and aqueous solution from the vapor in a scrubber which contains no rough surfaces or sharp edges, and which is preferentially oil wettable to prevent adherence of solids and to assure trouble-free operation.
18. Effect multi-stage distillation of aqueous solutions with sufficient control to permit separation of salts into various components in certain mineralized systems.
19. Convey heat from a remote, and possibly dilute, source by circulating oil through such a source to absorb heat, returning it to be mixed with aqueous solution for evaporation as herein disclosed; some of such heat sources being for example geothermal wells, sub-surface terrestrial formations, and solar heat absorption devices thereby utilizing clean and inexhaustible sources of natural heat. Should heated oil temperatures be below atmospheric pressure evaporation temperatures, subatmospheric pressures could be employed to effect evaporation using the afore-described methods.
20. Utilize an oil type liquid for the multi-purpose of conveying heat to an aqueous solution to be evaporated, wet the preferentially oil wettable surfaces of the confining system with a film to prevent adherence of solids and act as a vehicle to convey the suspended solids to a suitable settling zone offering the advantage of continuous, trouble-free operation.
21. Separate solids slurry into a saturated solution (for return to the system) and a damp solids product, using oil in the system of separation and handling to prevent adherence of solids to the preferentially oil wettable contacting surfaces thereby minimizing expensive shut-downs for cleaning.
22. Pre-heat the incoming raw aqueous solution, containing dispersed oil, with hot condensate product in a preferentially oil wettable heat exchanger thereby providing, continuous, maximum heat transfer without fouling of the surfaces.